1. Field of the Invention
The present invention is directed to a luminescent storage screen having a stimulable phosphor for the latent storage of x-ray images.
2. Description of the Prior Art
Luminescent storage screens are known in the art wherein a latent x-ray image is stored using a stimulable phosphor, with read-out of the x-ray image being achieved by exciting the phosphor with radiation of a first wavelength (stimulating radiation), which causes the phosphor to emit radiation of a second wavelength, which is acquired by a detector. Such a luminescent storage screen is disclosed, for example, in European Application 0 174 875.
A luminescent storage screen of this type employed in an image pick-up device is described in U.S. Pat. No. 3,859,527. In such an x-ray diagnostics installation, a luminescent storage screen, consisting of a luminescing stimulable phosphor which is irradiated with x-rays, is used as a radiation-sensitive transducer. Electronic holes are generated in the stimulable phosphor by the incident radiant intensity, these holes being stored in traps having a energy level, so that the latent x-ray image is stored in the screen.
For read-out, the entire area of surface of this screen, used as a master, is caused to luminesce pixel-by-pixel by an additional radiation source, which may be a laser. Due to the stimulating radiation, the energy of the holes stored in the traps is boosted and they can fall back into lower energy levels, whereby the energy difference is radiated in the form of light quanta. The stimulable phosphor thereby emits light dependent on the energy stored in the phosphor. The light emitted as a result of this stimulation is detected and rendered visible, so that the x-ray image which was latently stored in the screen can be read out.
A problem in the read-out of such conventional screens is that the stimulable phosphor is not sufficiently transparent for the laser light. A minimum thickness of the stimulable phosphor is required to be able to achieve adequate x-ray quantum absorptions. In the case of a non-transparent, tightly compressed or sintered phosphor, the laser beam is so greatly attenuated by the phosphor that the penetration depth of the laser beam is too small. Because the energy is no longer adequate for boosting the holes to the energy level required for quantum emission, the information stored in the deeper levels cannot be read out.
The storage screen disclosed in the European Application 0 174 875 has phosphor grains which are applied on a substrate enveloped by a binder. The binder serves the purpose of fixing the phosphor grains. A light-transmissive carrier material is usually employed as the binder, which is transparent both for the exciting laser light and for the emitted luminescent light. A problem associated with screens of this type, however, is that the laser beam spreads increasingly with increasing penetration depth, due to scattering of the beam at the phosphor grains, so that the modulation transfer function of the overall system is degraded. A storage screen in binder technology also has poorer quantum x-ray quantum absorption, compared to a layer of comparable thickness of the stimulable phosphor.
It is preferable, however, to vapor-deposit the stimulable phosphor onto a carrier in a high vacuum, and to temper the phosphor in a protective gas atmosphere, or in the vacuum, or to compress the phosphor under vacuum and/or heating, as disclosed in European Application 0 369 049. It is also possible to reshape single crystals of stimulable phosphor to the large area required for medical diagnostics by compressing such crystals. The latter methods yield transparent stimulable phosphor panels. The advantage of the transparency is that the stimulating laser beam cannot be spread in the storage medium due to scattering at the grains of the material. Such spreading of the read-out beam, as noted above, considerably degrades the modulation transfer function of the overall system. Spreading of the laser beam upon transirradiation of the storage medium is greatly diminished by using a transparent stimulable phosphor manufactured, for example, by compressing the phosphor powder.
The problem of reflection of the exciting, electromagnetic radiation having a first wavelength at the back side of the stimulable phosphor layer arises to a far greater degree than in the case of non-transparent layers. This problem is explained in detail with reference to FIG. 1. For pixel-by-pixel read-out of the x-ray image, the stimulating beam 6, having a first wavelength, penetrates the luminescent storage screen 1 which, for example, may be composed of a carrier and a binder having a stimulable phosphor applied thereon, or may be composed of a single crystal. The radiation is incident on the stimulable phosphor causing the phosphor to emit radiation 9, at a second wavelength, as a result of the above-described excitation. Upon emergence of the beam 6 from the storage screen 1, radiation 7, which in turn are incident or phosphor particles, are reflected back into the screen 1, and thus emit radiation 8, having the second wavelength, again due to excitation. The radiation 8 and the radiation 9 emerge from the storage screen 1, and are acquired by a detector (not shown). As a result, the detector also receives the radiation 8, which are allocated to different locations of the storage screen 1 than those which are to be read, the radiation 8 arriving at the detector either earlier or later than the radiation 9. Information in the form of radiation, which does not belong to the pixel currently being scanned by the beam 6, thereby degrades the resolution of the resultant image because the radiation 8 degrade the signal-to-noise ratio as background radiation.